At the time of writing, the K-Band beacon/transponder on AO-40 is
active on every orbit, from MA=118 to MA=138. The signals were first
heard a few dB above noise with a relatively modest system (a 22 x 18 cm
offset dish) with a ~2dB noise figure rx. A photo of this system is shown
below:

System used to receive first signals from A0-40 on S-Band
(click to enlarge)

The dish is sitting on top of our old tropo 24GHz transverter,
jacked up with blocks of wood to get the correct elevation. A 12V power
supply, IC202 and Workmate completed the system. The dish is fed with a
small rectangular WG20 horn, connected at the feedpoint to a DB6NT
waveguide input/output LNA. The preamp is connected to the
transverter by a short length of flexible waveguide.

The signals received on this system showed a large amount of
fading, owing to the use of linear polarisation on the satellite and its approx
5rpm axial spin. This fading plays havoc with trying to decode the
telemetry, and it was decided to construct a new feed horn to enable circular
polarisation to be used.

The new feedhorn consists of a short length of copper WG20, the
last inch of which was progressively squashed in a vice until the end had a near
rectangular aperture. The still almost-square corners were rounded off by
forcing an 8.5mm drill progressively into the open end, until it was close to
breaking through the sidewall. In this way, a (rough but adequate)
rectangular to circular transition was formed. A piece of scrap 12.7mm id
8.75mm id brass tubing about 40mm long was then soldered on to the end of the
WG20 transition, and a small conical horn made from thin brass sheet was
soldered on the other end, to complete the feedhorn.

The still-linear polarisation of this arrangement was converted
to circular using a dielectric polariser. This consists of a length of
1.6mm thick dielectric (obtained from a sheet of teflon pcb material with the
copper etched off) located in the tube section of the feedhorn. Dielectric
constant is unknown, but is probably about 2.2. The polariser is located
at 45 degrees to plane of input linear polarisation, and works by
delaying the component of the signal which is the the plane of the polariser by
90 degrees, while leaving the signal orthogonal to the plane of the polariser
unaffected. Emerging from the other end of the polariser are thus two orthogonal
signals of hopefully about the same amplitude, with a phase difference of 90
degrees. As these signals combine in space, the resultant is a circularly
polarised signal.

The polariser was designed empirically by cutting a length of
material, putting it into the waveguide and then checking for circularity of the
emerging signal. This was done by feeding a signal into the waveguide and
connecting a detector to a small waveguide horn about 60cm away (in this case
the now-discarded original feedhorn). The length of the polariser was
adjusted for minimum variation of the detector output as the sampling horn was
rotated on its axis. The length which worked best was 9.0mm, when the
emerging wave had less than 0.5dB variation. Getting the polariser at the
correct angle was important.

The position of the polariser in the circular tube section did
not seem to be critical (as expected). A position was found which seemed
to help the VSWR (which ended up at about 1.2).

Photos of the feedhorn are shown below:

Side view of the CP feedhorn (click to enlarge)

View down the mouth of the horn showing dielectric polariser
(click to enlarge)

In the upper photo, the large object between the preamp and the
horn is a plastic spacer made from the ends of an old wire bobbin sawn in half,
which serves to clamp the feedhorn into the feed support of the dish. The
concentric wires around the horn were used to hold the horn together during
soldering and serve no (known) electrical function.

In use, the new feedhorn with polariser was used in conjunction
with a larger dish (60cm offset) to compensate for the 3dB lost in going to
circular polarisation and made an enormous difference to the quality of the
signals. Spin fading is now not detectable, and CRC OK telemetry has
been decoded. The 60cm dish was mounted on a boom connected to the el/az
mount of our 3m dish used for S-Band, and carefully aligned using sun noise so
that the beams of the two antennas were as closely aligned as possible. In
the way, by peaking up the S-Band signals carefully, the K-Band signals can be
found, with only small antenna heading adjustments being necessary. The
arrangement is shown below:

(click to enlarge)

With this 60cm system, easy QSOs are possible with the K-Band
transponder and clean telemetry blocks can be decoded when squint is low and
there is not too much passband activity. The design of the transponder is
such that strong passband signals rob the beacon of some power, and it can then
drop below detection threshold. We use the AO40RCV pc sound card decoder
software, which is excellent for K-Band as it copes very well with minor
frequency wobbles and the large doppler shift variations. It can be
configured to tune the radio automatically, which is great as it takes away one
thing to worry about.

Since working with the 60cm dish for a few passes, it was
decided to try the 10ft dish on K-Band. The Andrews dish is quoted as
being useable to 30GHz, so it was worth trying. The dish has an f/D ratio
of 0.3, and initially we decided to feed it with the same feed as used with the
60cm dish, which as a much larger f/D ratio. The beamwidth of the
horn is much narrower than would be needed to fully illuminate the dish, with
the advantage that not all of the aperture would be used, leading to a wider
beamwidth and more likelihood of being able to find the satellite (our el/az
readouts are rather crude and insufficient to point the antenna
"blind"). The horn was fitted into the S-Band helix feed
so that the dish can be used on both bands simultaneously, so we could use the
S-Band to pre-align the heading. This has worked out very well, so that
the K-Band signals are at least detectable after peaking up on S-Band.

In use on a few passes, the big dish is working well. Sun
noise is up from 5db with the 60cm to near 11dB, transponder noise is now
detectable (about 3dB average) and beacon/passband are now quite strong.
Telemetry decoding is practically solid at low squint angles, provided the dish
is kept on heading! The software seems to be able to cope with the
fluctuation of the beacon level with passband activity.

K-Band horn is located centrally within the helix and is at
the (guessed) phase centre of the helix (click to enlarge)

Below are a some P3T windows:

(click to enlarge)

(click to enlarge)

Here is examples of some telemetry received on K-Band with
the 10ft dish:
tlm file and audio (6 good blocks in sequence,
approx 1.4MB!)
Listen to a strong signal via the K-Band transponder
(ON4DY).

A Study of the AO-40 K-Band Beacon Level v Squint Angle

During March and April 2002, the signal strength of the
K-Band Beacon has been measured on every fifth orbit between MA=110 and
114. During this period the passband has been switched off, allowing all
available power to be used for the beacon. With the transponder on, the
beacon level can vary considerably depending on the level of activity in the
50kHz passband (which is centred on the beacon). Observations were only logged
when the sky was clear, to try to reduce any variations caused by cloud
absorption. System performance was checked regularly during the monitoring
period using sun noise.

The system used a 10ft dish with a 30GHz rated surface accuracy.
A high gain horn was used, to under-illuminate the dish to help finding the
beacon, by increasing the beamwidth. The feedhorn incorporates a
polariser, to convert the linear polarisation of the rectangular waveguide into
circular polarisation, with approximately +/- 0.5dB circularity. This
means that 3dB of signal was thrown away, as the satellite antenna is linearly
polarised. The advantage was that the polarisation-induced fading was
eliminated. A home-brew downconverter based on the DB6NT Mk 2 transverter and a
DB6NT 3 stage waveguide preamp completed the system.

Despite the polariser, the signal still shown some spin-induced fading, which
gets worse at larger squint angles. This can make the beacon difficult to
find when the squint is high (uncertainties of dish heading, frequency and
whether the beacon is at the peak of a fade!).
The plots below show the results. The data points are the large diamonds,
and the line is a "curve fit" to the data. The shape may or may
not be meaningful - more data points will be added as time permits, and
published at www.g3wdg.free-online.co.uk/kband.htm
. Details of the polariser can also be found at this URL.

As noted above, these measurements were all made with the
transponder passband switched off. With the passband on, initial
measurements show a drop of about 3dB in the strength of the beacon (with no
signals in the passband). Also, about 3dB of passband noise was
detected (at about 3 degrees squint).

Level v Squint
(click to
enlarge)
Fade v squint (click to enlarge)

A Waveguide Cavity Filter for 24GHz.

This filter is used in my receive system with a 144MHz
IF. Details are in the pictures (click each to enlarge).